1) Cellular revertants resistant to transformation by specific oncogenes, were characterized. Revertants were originally derived from mutogenized populations of Kirsten sarcoma virus (Ki-Mu SV)-transformed NIH/3T3 cells and certain contain 2 copies of the v-ki-ras gene, elevated levels of p 21 and rescuable Ki-Mu SV capable of transforming susceptible cells. Retransformation frequences were measured in the revertant cells following infection with a variety of retroviruses with the following results: v-ki-ras and the related viral oncogenes v-Ha-ras, as well as the activated human oncogene c-Ha-ras (human) failed to transform the revertant cell lines. V-fes was also negative for retransformation. Oncogenes which were able to retransform the revertant cell lines included v-fms, v-sis, and v-moz as well as polyoma virus. The v-fes gag gene product was present in elevated amounts in fes-transformed cells after fusion to the revertant cell lines, even though such hybrids has lost their transformed morphology. Initial experiments also showed that revertants produced high levels of sarcoma growth factor capable of stimulating anchorage-independent along formation by test cells. 2) A chimeric DNA construction consisting of the v-Ha-ras gene ligated to the LTR sequence of MMTV has resulted in transfectants which are dependent on the presence of glucocorticoid for expression of the transformed phenotype.